1. Field of the Invention
The present invention relates to lens position control in an imaging apparatus such as a video camera and a digital still camera.
2. Related Art
In general, a lens device having a zooming function and a focusing function used in a video camera and a digital still camera has a structure which moves respective units, such as a zooming lens unit and a focusing lens unit, disposed inside a lens barrel independently along an optical axis to adjust focus and the like.
FIG. 12 shows one example of a structure of a lens system. A fixed lens 1001 is secured to a lens barrel 1000. A zoom lens 1002 having a function for adjusting the zoom magnification and a focusing lens 1003 having a focus-adjusting function are allowed to move inside the lens barrel 1000 in parallel with an optical axis. Optical information inputted through the respective lenses 1001, . . . is inputted to an imaging device 1004. Here, the lens that is moved in parallel with the optical axis (zoom lens 1002 or focusing lens 1003) is defined as a lens unit. In order to move the lens unit to a target position, it is necessary to accurately detect a reference position that is used as a reference for controlling the movement. For this reason, conventionally, various reference position detecting methods have been developed.
For example, with respect to the reference position detection, there are proposed a method using a non-contact type sensor, such as a photo-interrupter and a Hall element and a method using a contact-type sensor, such as a leaf switch (for example, JP-A-2001-324664). A conventional reference position detecting method using a light-shielding member attached to the lens unit and the photo-interrupter is described below.
FIG. 13 is a block diagram showing a position-detecting device for a lens unit, which uses a photo-interrupter. A lens unit 1101 is moved in an optical axis direction by a driving motor 1106. The driving motor 1106 is driven by a motor driver 1107. The reference position of the lens unit 1101 is detected by a reference position detector 1103 by using a signal from a photo-interrupter 1102. A light-shielding member 1105 is attached to the lens unit 1101. The light-shielding member 1105 traversing the photo-interrupter 1102 allows the position of the lens unit 1101 to be detected.
A lens drive command unit 1108 outputs a command for driving a lens (referred to as “a lens drive command”) to the motor driver 1107 in accordance with the lens drive command value provided by the reference position detector 1103. The reference position detected by the reference position detector 1103 is stored in a reference position storing unit 1110.
FIG. 14A shows detail configuration of the photo-interrupter 1102 and the light-shielding member 1105. The photo-interrupter 1102 includes a light-emitting element 1201 and a light-receiving element 1202. The photo-interrupter 1102 is disposed at a predetermined position defining a reference position. When the lens unit 1101 is moved along the optical axis in parallel therewith, the light-shielding member 1105 attached to the lens unit 1101 is moved together with the lens unit 1101. When the light-shielding member 1105 crosses the space between the light-emitting element 1201 and the light-receiving element 1202, the light emitted from the light-emitting element 1201 to the light-receiving element 1202 is shielded by the light-shielding member 1105 so that the output signal level of the photo-interrupter 1102 is changed. The reference position of the lens unit 1101 is detected by monitoring the change in the output signal level of the photo-interrupter 1102.
FIG. 14B shows a change in the output signal level of the photo-interrupter 1102. In this Figure, in a state where the light-shielding member 1105 is not located between the light-emitting element 1201 and the light-receiving element 1202 (hereinafter, referred to as a “complete transmissive state (1301)”), the output voltage of the photo-interrupter 1102 has a maximum value. After the light-shielding member 1105 is moved from the complete transmissive state (area 1301) by driving the lens unit 1101, and when the light-shielding member 1105 starts crossing the space between the light-emitting element 1201 and the light-receiving element 1202, the output voltage of the photo-interrupter 1102 starts dropping. When the light-shielding member 1105 comes to a state (hereinafter, referred to as a “complete shield state (area 1302)”) which completely shields the light emitted from the light-emitting element 1201 to the light-receiving element 1202, the output voltage of the photo-interrupter 1102 has a minimum value. The area 1301 forming the complete transmissive state is referred to as a “complete transmissive area”, and the area 1302 forming the complete shield state is referred to as a “complete shield area”.
Suppose that a state in which the output signal level of the photo-interrupter 1102 exceeds a predetermined threshold value 1303 is referred to as “H” and a state in which the output signal level of the photo-interrupter 1102 is less than the predetermined threshold value 1303 is referred to as “L”. The threshold value 1303 is set to a level between the output signal level of a complete transmissive state (1301) and the output signal level of a complete light-shield state (1302). The reference position detector 1103 continuously monitors the state of the output signal level of the photo-interrupter 1102 during a period from the start of the reference position detection until the reference position has been fixed. In response to a change in the state of the output signal level of the photo-interrupter 1102 thus acquired, the reference position detector 1103 calculates and sets a lens drive command value, and transmits the lens drive command value to the lens drive command unit 1108. A reference position of the lens is fixed to a position at which the output signal level of the photo-interrupter 1102 switches from “L” to “H”.
The reference position of the lens unit 1101 needs to be determined accurately. Therefore, conventionally, the lens unit 1101 is moved in two directions, that is, from the fixed lens 1001 side (hereinafter, referred to as “forward”) as well as from the imaging device 1004 side (hereinafter, referred to as “backward”), relative to the attached position of the photo-interrupter 1102 so as to detect the reference position. By carrying out the reference position detection in the two directions, that is, forward and backward, it is possible to accurately detect the reference position even when the stop position of the lens unit 1101 is undesirably shifted due to an external influence such as an impact.
However, in the above-mentioned structure in which the reference position detection is carried out in two directions, that is, forward and backward, it takes much time to fix or determine the reference position. Moreover, when it takes much time to detect the reference position as described above, a problem arises in which there is a delay in activating an imaging apparatus such as a video camera and a digital still camera.
The present invention has been devised so as to solve the above-mentioned problems, and its objective is to provide a position detecting device capable of reducing time required for detecting the reference position and detecting the reference position accurately even upon occurrence of an external influence such as an impact.